Glass-ceramics process for their preparation and use

ABSTRACT

The present invention refers to glass-ceramics consisting of the mixtures (I): ZrO 2 —SiO 2 —Me II O, or (II) SiO 2 —Me III O 2 —Me II O, wherein: Me II  is chosen in the group consisting of: Ca, Ba, Mg, Zn or mixture thereof; Me III  is chosen in the group consisting of Al, B or mixtures thereof; each of the above said constituents being present in determined quantities; the invention refers also to a process for preparing the glass-ceramics above defined; porcelain stonewares and glazes containing them and their use for preparing ceramic items.

This is a division of Ser. No. 09/485,270, filed Feb. 7, 2000 now U.S.Pat. No. 6,605,554 which claims priority from International ApplicationNo. PCT/EP97/04387, filed Aug. 11, 1997.

FIELD OF THE INVENTION

The present invention refers to glass-ceramic consisting of the mixtures(I) or (I):ZrO₂—SiO₂—Me″O  (I)SiO₂—Me₂′″O₃  (II)wherein in mixture (I):

-   Me″=Ca, Ba, Mg, Zn or mixtures thereof:-   and the percentage, in weight, for each component is:-   ZrO₂ 5-25%-   SiO₂ 45-75%-   Me″O 15-45%    and in mixture (II)-   Me″O is as above defined;-   Me′″=Al, B or mixtures thereof    and the percentage in weight for each of the above said components    is:-   SiO₂ 30-65%-   Me′″O₃ 5-25%-   Me″O 5-40%

STATE OF THE ART

It is known that glass is an amorphous material obtained by melting ofcrystalline compounds followed by cooling down of the melted mass.

On the contrary glass-ceramics (hereinafter indicated as GC) arevitreous systems that, when brought to a temperature T₁ higher thentheir glass transition temperature T_(g), present the formation ofcrystal nuclei (homogeneous or heterogeneous) with following crystalgrowth.

Porcelain stoneware (also defined as ceramic body having absorption</=0.5%, according to ISO 13006 annnex B1A) is a ceramic materialprepared starting from natural crystalline products which, submitted toa syntherization process, partially melt and are transformed into newcrystalline phases. This products are prepared starting from a mixtureof clay minerals, fondents and possibly eutectic promoters. Glaze is aceramic product consisting of fondents and silica based glasses whichare grinded in granules of the wanted dimensions and thereafter appliedon the appropriate substrate and heated so that the grinded granulesmelt (totally or partially) covering the substrate surface.

All the above said products, having a vitreous surface, conferimpermeability and higher physico-chemical properties (better resistanceto chemical agents, abrasion etc.) to porous substrates. Moreover theyplay a very important role as aesthetic materials due to the use of leadbased fondents, opacifiers (represented by dispersions of, for example,tin oxide or zirconium silicate which are added in the form of crystalshaving a determined granulometric dimensions) and coloured pigments.

However the known materials do not completely satisfy the market needsin so far as the physico-chemical properties are concerned and alsotheir opacization (and the consequent white index) is not alwayssatisfactory giving final products which are aesthetically not suitablefor the appropriate use; moreover the known processes for the productionof glass-ceramics involve high costs and waste of energy.

DETAILED DESCRIPTION OF THE INVENTION

The invention solves the above said problems thanks to new glass-ceramichaving the composition reported above.

The glass-ceramic according to the invention can be used as such inorder to obtain ceramics or can be added to the materials usuallyemployed for preparing porcelain stoneware or glazes.

The use of the glass-ceramics according to the invention allows (thanksto their “in situ” crystallization) to confer to glaze an exceptionalopacization which results in an higher white index (WI) compared to thatobtained with the normal opacifiers added to glass, moreover, thanks tothe chemical stability and high mechanical resistance of the crystallinephase recrystallized on the glaze surface, they confer to the substratephysico-chemical properties higher then those of the traditional glazes.

The glass-ceramics according to the invention allow also the preparationof ceramics directly from the melted mass which can be shaped in thedesired form or the preparation of the wanted ceramic items by tapecasting of slurries or hot and cold pressing of the powders.

In the following TABLE I preferred glass-ceramic according to theinvention are reported (the percentage of the components is given inweight):

TABLE 1 Comp. SiO2 CaO ZrO2 BaO ZnO MgO Al2O3 B2O3 GC.1 55 33 12 GC.252.5 31.3 16.2 GC.3 55 21 12 12 GC.4 52.3 21.9 16.4 9.4 GC.5 55 11 12 1111 GC.6 51.6 39.7 8.7 GC.7 37.8 17.63 37.98 6.59 GC.8 47.95 23.55 9.319.2 GC.9 41 0.5 30 20.5 8 GC.10 36.77 47.05 9.75 6.43 GC.11 61 24 15

The glass-ceramics according to the present invention can be preparedaccording to a process which is substantially similar to the onefollowed for the production of porcelain stoneware consisting in thegrinding/mixing/pressing (and following firing of the pressed materials)of powders with the difference that in this case the powders consistsonly of glass (i.e. they lack the crystalline starting materials usedfor the production of porcelain stoneware).

However, it is essential that the firing step is performed according towell defined and controlled thermic cycles in order to develop thewanted properties.

In particular, once the T_(g) (transition temperature) and the T_(c)(crystallization temperature/s) of the material are determined(according to usual methods) the thermic cycle must be performed asfollows:

starting from room temperature the powder mixture is heated increasingthe temperature by 10°-30° C. per minute up to 350° C. (in order toeliminate the organic impurities according to usual processes)thereafter the heating temperature is increased by 10°-30° C. per minuteup to the T_(g) maintaining the temperature at this value for 0-120′,thereafter the temperature is increased by 10°-30° C. per minute up tothe T_(c) were it is maintained for 0-several hours, for example up toseven hours, preferably for 0-4 hours, and possibly increased by 10°-30°C. up to the next T_(c) and so on up to the final T_(c); once completedthe heating (i.e. once the highest T_(c) has been reached) the mass iscooled down to room temperature.

As above said the glass-ceramics according to the invention can be addedto the materials normally used for the production of porcelainstoneware; the adding of the glass-ceramics according to the inventionallows the production of ceramics via the process normally used for theproduction of porcelain stoneware this resulting in an important savingof energy.

For example a glass-ceramic according to the invention is added to atraditional mixture of starting materials for the production ofporcelain stoneware; the mixture is loaded in a Alsing ball-mill(continuous or discontinuous) and grinded in the presence of water togive a slurry which is dried in an atomizer forming hollow grains. Thegrains are shaped in the form of tiles by traditional pressing. Thetiles are dried and fired in a traditional furnace at 900°-1230° C.giving the final product. The obtained tiles are constituted ofsyntherized and crystallized materials and a residual vitreous phase asdemonstrated by mineral and X-ray analysis of the tile surface.

Preferably the glass-ceramics according to the invention are added tothe traditional materials in a quantity comprised between 5% and 65% (inweight). In TABLE 2 it is reported (expressed in metal oxide percentagein weight) an example of the composition of a mixture of startingmaterials which is added to the glass-ceramics of the present inventionin the above reported quantities:

TABLE 2 SiO₂  40-80 Al2O3   5-40 MgO 0.1-10 CaO 0.1-10 Na2O 0.1-10 K2O0.1-10

In this connection it was surprisingly found, and it is another objectof the present invention, that also other glass-ceramics, already known“per se”, can be added to the traditional starting materials forproduction of porcelain stoneware giving similar advantageous results.

Examples of already known glass-ceramics (a-d) which can be used for theabove said purpose are given in following TABLE 3 (the percentage isexpressed in weight).

TABLE 3 Glass- ceramic SrO Al2O3 SiO2 ZrO K2O MgO a 30-40 25-30 30-45 b40-50 10-25 30-40 c 15-25 60-70 10-20 d 20-25 10-25 45-55  5-15

The glazes can be produced via the processes traditionally used for theproduction of glazes. A glass-ceramic according to the invention and astarting material usually employed for the production of ceramic glazes(which are essentially the same reported for the preparation ofporcelain stoneware with the addition of fondents like frits or boratesor lead oxide etc.) were loaded in the appropriated proportions in aAlsing ball-mill in the presence of water and grinded to obtain a socalled “glaze” which was applied by airbrush, threading die, bell orserigraphy on a traditional substrate, crude or biscuited, obtained bypressing of atomized or dry-grinded powders. The glaze, dried and/orgranulated, can be deposited on the substrate surface by falling andfixed on the surface with appropriated ligants. The substrate is firedat 900°-1230° C. in a quick- or tunnel-furnace (continuous ordiscontinuous) giving the final product in which the glass-ceramic hasinduced a controlled crystallization. The glazed tiles so obtainedpresent therefore a vitreous and a crystallized part which confer a verywell defined microstructure as shown by SEM and X-rays diffractometry.

Preferably the glass-ceramics according to the invention are added tothe traditional materials in a quantity comprised between 5% and 60% (inweight).

EXAMPLE 1 Preparation of Porcelain Stoneware (Corresponding to Example 2in TABLE 4)

GC2 (50% of the total weight) is loaded in a discontinuous Alsingball-mill together with the traditional starting materials (see ex.2 inTABLE 4) (50% of the total weight).

Water (up to 50% in weight of the material loaded) and 0.4% in weight ofsodium tripolyphosphate (as fluidizer) are added.

The mixture is grinded until the slurry residue on a 63 micron sieve is0.7-1% (in weight).

The slurry is poured into a tank under mechanical stirring andthereafter is spray dried leaving about 6% of water which is the idealquantity for the following pressing operation.

The humid powder is pressed at 250-500 kg/cm² in the wanted shapes andheated in a furnace at 1230° C.

By repeating the process described in Example 1 but using the productsand the quantities indicated in the following TABLES 4-13 otherporcelain stonewares were obtained; the glass-ceramics are indicatedmaking reference to TABLE 1 and the quantities are expressed in % inweight:

TABLE 4 Glass-ceramics according to % Glass- % Traditional TraditionalMaterials invention ceramics materials SiO₂ 76.0 GC2  5 95 Al₂O₃ 17.8MgO 0.8 CaO 1.0 Na₂O 2.3 K₂O 2.1 SiO₂ 54.6 GC2 50 50 Al₂O₃ 41.8 MgO 0.2CaO 0.7 Na₂O 0.4 K₂O 2.3 SiO₂ 54.5 GC2 65 35 Al₂O₃ 42.5 MgO 0.2 CaO 0.5Na₂O 0.3 K₂O 2.0

TABLE 5 Glass-ceramics according to % Glass- % Traditional TraditionalMaterials invention ceramics materials SiO₂ 73.5 GC3  5 95 Al₂O₃ 18.1MgO 0.8 CaO 1.0 Na₂O 3.5 K₂O 3.1 SiO₂ 84.0 GC3 50 50 Al₂O₃ 10.3 MgO 0.3CaO 0.7 Na₂O 2.4 K₂O 2.3 SiO₂ 56.3 GC3 65 35 Al₂O₃ 39.5 MgO 0.3 CaO 0.6Na₂O 1.3 K₂O 2.0

TABLE 6 Glass-ceramics according to % Glass- % Traditional TraditionalMaterials invention ceramics materials SiO₂ 74.8 GC4  5 95 Al₂O₃ 20.1MgO 0.7 CaO 1.1 Na₂O 1.2 K₂O 2.1 SiO₂ 61.3 GC4 50 50 Al₂O₃ 35.3 MgO 0.3CaO 0.1 Na₂O 1.2 K₂O 1.8 SiO₂ 60.0 GC4 65 35 Al₂O₃ 37.0 MgO 0.2 CaO 0.8Na₂O 1.0 K₂O 1.0

TABLE 7 Glass-ceramics according to % Glass- % Traditional TraditionalMaterials invention ceramics materials SiO₂ 72.2 GC5  5 95 Al₂O₃ 18.5MgO 0.9 CaO 1.3 Na₂O 4.0 K₂O 3.1 SiO₂ 81.5 GC5 50 50 Al₂O₃ 12.0 MgO 0.2CaO 1.0 Na₂O 2.2 K₂O 3.1 SiO₂ 58.5 GC5 65 35 Al₂O₃ 37.3 MgO 0.3 CaO 0.6Na₂O 1.0 K₂O 2.3

TABLE 8 Glass-ceramics according to % Glass- % Traditional TraditionalMaterials invention ceramics materials SiO₂ 76.0 GC9  5 95 Al₂O₃ 17.8MgO 0.8 CaO 1.0 Na₂O 2.3 K₂O 2.1 SiO₂ 62.8 GC9 50 50 Al₂O₃ 25.8 MgO 0.2CaO 0.9 Na₂O 7.1 K₂O 3.2 SiO₂ 63.6 GC9 65 35 Al₂O₃ 31.6 MgO 0.3 CaO 1.1Na₂O 1.0 K₂O 2.4

TABLE 9 Glass-ceramics according to % Glass- % Traditional TraditionalMaterials invention ceramics materials SiO₂ 75.9 GC6  5 95 Al₂O₃ 17.7MgO 0.8 CaO 1.0 Na₂O 2.5 K₂O 2.1 SiO₂ 72.8 GC6 50 50 Al₂O₃ 10.1 MgO 6.7CaO 0.2 Na₂O 7.5 K₂O 2.7 SiO₂ 67.0 GC6 65 35 Al₂O₃ 20.0 MgO 2.2 CaO 0.4Na₂O 7.8 K₂O 2.6

TABLE 10 Glass-ceramics according to % Glass- % Traditional TraditionalMaterials invention ceramics materials SiO₂ 74.0 GC7  5 95 Al₂O₃ 14.2MgO 1.5 CaO 1.5 Na₂O 4.2 K₂O 4.6 SiO₂ 72.6 GC7 50 50 Al₂O₃ 14.8 MgO 1.3CaO 2.0 Na₂O 9.0 K₂O 0.3 SiO₂ 65.9 GC7 65 35 Al₂O₃ 23.6 MgO 0.2 CaO 0.9Na₂O 9.1 K₂O 0.3

TABLE 11 Glass-ceramics according to % Glass- % Traditional TraditionalMaterials invention ceramics materials SiO₂ 74.3 GC8  5 95 Al₂O₃ 15.8MgO 1.5 CaO 2.2 Na₂O 3.7 K₂O 2.5 SiO₂ 68.0 GC8 50 50 Al₂O₃ 26.8 MgO 2.2CaO 0.4 Na₂O 1.0 K₂O 1.6 SiO₂ 67.0 GC8 65 35 Al₂O₃ 28.0 MgO 1.8 CaO 1.2Na₂O 0.3 K₂O 1.7

TABLE 12 Glass-ceramics according to % Glass- % Traditional TraditionalMaterials invention ceramics materials SiO₂ 74.0 GC10  5 95 Al₂O₃ 17.6MgO 1.8 CaO 1.4 Na₂O 3.7 K₂O 1.5 SiO₂ 69.0 GC10 50 50 Al₂O₃ 26.8 MgO 1.2CaO 0.4 Na₂O 1.0 K₂O 1.6 SiO₂ 67.9 GC10 65 35 Al₂O₃ 28.5 MgO 1.0 CaO 1.1Na₂O 0.4 K₂O 1.1

TABLE 13 Glass-ceramics according to % Glass- % Traditional TraditionalMaterials invention ceramics materials SiO₂ 78.0 GC11  5 95 Al₂O₃ 10.8MgO 2.2 CaO 0.4 Na₂O 6.0 K₂O 2.6 SiO₂ 70.1 GC11 50 50 Al₂O₃ 14.9 MgO 8.7CaO 0.1 Na₂O 4.3 K₂O 1.9 SiO₂ 69.0 GC11 65 35 Al₂O₃ 27.0 MgO 1.0 CaO 1.1Na₂O 1.0 K₂O 0.9

EXAMPLE 2 Preparation of Glazes (Corresponding to Example 1 in TABLE 14)

GC2 (30% in weight) was loaded in a discontinuous Alsing ball-milltogether with the traditional starting materials (see Ex.1 in TABLE 14)(50% of the total weight). Water (up to 50% in weight of the totalmaterial loaded), sodium tripolyphosphate (as fluidizer) (0.4% inweight) and hydroxymethylcellulose (0.3%) (as ligant) are added.

The mixture is grinded until the slurry residue on a 16000 micron sieveis about 2% (in weight).

The slurry is bell applied (500 g-2.5 Kg) on a pressed substrate whichis heated in a furnace at 1160° C.

By repeating the process-described in Example 2 but using the productsindicated in the following TABLES 14-23 other glazes were prepared; theglass-ceramics are indicated making reference to TABLE 1 and thequantities are expressed in % in weight.

In each TABLE are reported the starting materials and the correspondingquantities for the preparation of two different glazes (indicated asGlaze I and Glaze II) starting from the same glass-ceramic according tothe invention and the same traditional starting compounds but usingdifferent quantities of the same.

TABLE 14 Traditional starting materials and/or frits Glass-ceramic GlazeI Glaze II GC2 30 50 SiO₂ 54.0 70 50 Al₂O₃ 33.0 MgO 0.3 CaO 0.7 Na₂O 8.0K₂O 4.0

TABLE 15 Traditional starting materials and/or frits Glass-ceramic GlazeI Glaze II GC3 30 50 SiO₂ 60.0 70 50 Al₂O₃ 31.8 MgO 0.2 CaO 0.8 Na₂O 6.2K₂O 1.0

TABLE 16 Traditional starting materials and/or frits Glass-ceramic GlazeI Glaze II GC4 30 50 SiO₂ 60.0 70 50 Al₂O₃ 31.2 MgO 0.4 CaO 0.8 Na₂O 6.0K₂O 1.6

TABLE 17 Traditional starting materials and/or frits Glass-ceramic GlazeI Glaze II GC5 30 50 SiO₂ 60.0 70 50 Al₂O₃ 28.0 MgO 0.0 CaO 5.0 Na₂O 3.0K₂O 2.0 SnO₂ 2.0

TABLE 18 Traditional starting materials and/or frits Glass-ceramic GlazeI Glaze II GC9 30 50 SiO₂ 58.0 70 50 Al₂O₃ 18.0 MgO 2.5 CaO 10.5 Na₂O6.0 K₂O 5.0

TABLE 19 Traditional starting materials and/or frits Glass-ceramic GlazeI Glaze II GC6 30 50 SiO₂ 59.0 70 50 Al₂O₃ 10.0 MgO 1.0 CaO 7.0 Na₂O 8.0K₂O 3.0 ZrO₂ 12.0

TABLE 20 Traditional starting materials and/or frits Glass-ceramic GlazeI Glaze II GC7 30 50 SiO₂ 60.0 70 50 Al₂O₃ 17.1 MgO 2.4 Na₂O 6.2 K₂O 4.3ZrO₂ 8.0 TiO₂ 2.0

TABLE 21 Traditional starting materials and/or frits Glass-ceramic GlazeI Glaze II GC8 30 50 SiO₂ 54.0 70 50 Al₂O₃ 26.0 MgO 0.1 CaO 8.5 Na₂O11.0 K₂O 0.2 TiO₂ 0.2

TABLE 22 Traditional starting materials and/or frits Glass-ceramic GlazeI Glaze II GC10 30 50 SiO₂ 62.0 70 50 Al₂O₃ 6.3 MgO 1.3 CaO 14.0 Na₂O0.5 K₂O 5.6 ZnO 10.3

TABLE 23 Traditional starting materials and/or frits Glass-ceramic GlazeI Glaze II GC11 30 50 SiO₂ 50.0 70 50 Al₂O₃ 25.0 MgO 10.0 Na₂O 7.0 K₂O6.0 TiO₂ 2.0

EXAMPLE 3 Preparation of Glass-Ceramic

In this case the powder is prepared according to Example 1 but only GC2is used. The tile according to Example 1 is prepared applying thethermic cycle C1 reported in TABLE 24 wherein the T_(g) and the T_(c1)and T_(c2) of the starting materials (GC2) are also indicated.

In TABLE 25 thermic cycles for the preparation of compound GC8 arereported; the T_(g) and T_(c) of GC8 are also indicated.

All the Glass-ceramics obtained via the described processes showed atypical superficial micro-structure presenting several crystalline andvitreous phases.

TABLE 24 GC 2 Starting product T_(g) = 825° C. T_(c1) = 966° C. T_(c2) =1025° C. Thermic cycles applied: B1 C1 C2 Step total Time T Step TotalTime Step Total Time min (min) (° C.) min 1 (min) T (° C.) min (mm) T (°C.) 0 0 25 0 0 25 0 0 25 35 35 350 35 35 350 35 35 350 30 65 350 30 65350 30 65 350 55 120 900 55 120 900 55 120 900 30 150 900 30 150 900 30150 900 8 158 980 8 158 980 25 175 1150 30 188 980 30 188 980 30 2051150 17 205 1150 22 210 1200 30 205 1150 30 235 1150 30 240 1200

TABLE 25 GC8 starting product T_(g): 740° C. T_(c) 934° C. Thermiccycles applied: B1 C1 C2 Step Total Time T Step Total Time Step TotalTime min (min) (° C.) min (min) T (° C.) min (min) T (° C.) 0 0 25 0 025 0 0 25 35 35 350 35 35 350 35 35 350 30 65 350 30 65 350 30 65 350 44109 790 44 109 790 44 109 790 30 139 790 30 139 790 30 139 790 16 155950 26 165 1050 36 175 1150 30 185 950 30 195 1050 30 205 1150

1. Glass-ceramics consisting of the mixture:ZrO₂—SiO₂—Me″O wherein: Me″ is chosen in the group consisting of: Ca,Ba, Mg, Zn or mixtures thereof and the percentage (in weight) for eachcomponent is: ZrO₂=5-25% SiO₂=45-75% Me″O=15-45% where saidglass-ceramics are in the form of powders obtained by grinding. 2.Glass-ceramics according to claim 1 having the following composition (%in weight): SiO₂—CaO—ZrO₂ (55:33:12) SiO₂—CaO—ZrO₂ (52.5:31.3:16.2:)SiO₂—CaO—ZrO₂—BaO (55:21:12:12) SiO₂—CaO—ZrO₂—ZnO (52.3:21.9:16.4:9.4)SiO₂—CaO—ZrO₂—BaO—MgO (55:11:12:11:11.


3. Porcelain stoneware consisting of a glass-ceramic according to claim1 and a mixture of compounds used in the production of porcelainstoneware which comprise SiO₂  40-80 wt. % Al₂O₃   5-40 wt. % MgO 0.1-10wt. % CaO 0.1-10 wt. % Na₂O 0.1-10 wt. % K₂O  0.1-10 wt. %.


4. Porcelain stoneware according to claim 3 wherein the glass-ceramicrepresents 5%-65% in weight of the total weight.
 5. Porcelain stonewareconsisting of a crystalline powder which is a glass-ceramic thatrepresents 5%-65% in weight of the total weight having the chemicalformulaZrO₂—SiO₂—Me″O wherein: Me″ is chosen in the group consisting of: Ca,Ba, Mg, Zn or mixtures thereof and the percentage (in weight) for eachcomponent is: ZrO₂=5-25% SiO₂=45-75% Me″O=15-45% in admixture with amixture of metal oxides comprising SiO₂ (40-80 wt. %), Al₂O₃ (5-40 wt.%), MgO (0.1-10 wt. %), CaO (0.1-10 wt. %), Na₂O (0.1-10 wt. %) and K₂O(0.1-10 wt. %), whereby said crystalline powder represents 5% to 65% byweight of said Porcelain stoneware.
 6. Porcelain stoneware consisting ofthe following compositions (in weight): 5% of a glass ceramic accordingto claim 2 having the following composition:SiO₂—CaO—ZrO₂(52.5:31.3:16.2:) and 95% of a material consistingessentially of SiO₂ 76 wt. %; Al₂O₃ 17.8 wt. %; MgO 0.8 wt. %; CaO 1.0wt. % Na₂O 2.3 wt. %; K₂O 2.1 wt. %.
 7. Porcelain stoneware consistingof the following compositions (in weight): 50% of glass ceramicaccording to claim 2 having the following composition:SiO₂—CaO—ZrO₂(52.5:31.3:16.2:) and 50% of a mixture of traditionalmaterial consisting essentially of SiO₂ 54.6 wt. %; Al₂O₃ 41.8 wt. %;MgO 0.2 wt. %; CaO 0.7 wt. %; Na₂O 0.4 wt. % and K₂O 2.3 wt. %. 8.Porcelain stoneware consisting of the following compositions (inweight): 65% of glass ceramic according to claim 2 having the followingcomposition:SiO₂—CaO—ZrO₂ (52.5:31.3:16.2:) and 35% of a mixture of traditionalmaterial consisting essentially of SiO₂ 54.5 wt. %; Al₂O₃ 42.5 wt. %;MgO 0.2 wt. %; CaO 0.5 wt. %; Na₂O 0.3 wt. % and K₂O 2.0 wt. %. 9.Porcelain stoneware consisting of the following compositions (inweight): 5% of a glass ceramic according to claim 2 having the followingcomposition:SiO₂—CaO—ZrO₂—BaO (55:21:12:12) and 95% of a mixture of traditionalmaterial consisting essentially of SiO₂ 73.5 wt. %; Al₂O₃ 18.1 wt. %;MgO 0.8 wt. %; CaO 1.0 wt. %; Na₂O 3.5 wt. % and K₂O 3.1 wt. %. 10.Porcelain stoneware consisting of the following compositions (inweight): 50% of glass ceramic according to claim 2 having the followingcomposition:SiO₂—CaO—ZrO₂—BaO (55:21:12:12) and 50% of a mixture of traditionalmaterial consisting essentially of SiO₂ 84.0 wt. %; Al₂O₃ 10.3 wt. %;MgO 0.3 wt. %; CaO 0.7 wt. %; Na₂O 2.4 wt. % and K₂O 2.3 wt. %. 11.Porcelain stoneware consisting of the following compositions (inweight): 65% of glass ceramic according to claim 2 having the followingcomposition:SiO₂—CaO—ZrO₂—BaO (55:21:12:12) and 35% of a mixture of traditionalmaterial consisting essentially of SiO₂ 56.3 wt. %; Al₂O₃ 39.5 wt. %;MgO 0.3 wt. %; CaO 0.6 wt. %; Na₂O 1.3 wt. % and K₂O 2.0 wt. %. 12.Porcelain stoneware consisting of the following compositions (inweight): 5% of a glass ceramic according to claim 2 having the followingcomposition:SiO₂—CaO—ZrO₂—ZnO (52.3:21.9:16.4:9.4) and 95% of a mixture oftraditional material consisting essentially of SiO₂ 74.8 wt. %; Al₂O₃20.1 wt. %; MgO 0.7 wt. %; CaO 1.1 wt. %; Na₂O 1.2 wt. % and K₂O 2.1 wt.%.
 13. Porcelain stoneware consisting of the following compositions (inweight): 50% of glass ceramic according to claim 2 having the followingcomposition:SiO₂—CaO—ZrO₂—ZnO (52.3:21.9:16.4:9.4) and 50% of a mixture oftraditional material consisting essentially of SiO₂ 61.3 wt. %; Al₂O₃35.3 wt. %; MgO 0.3 wt. %; CaO 0.1 wt. %; Na₂O 1.2 wt. % and K₂O 1.8 wt.%.
 14. Porcelain stoneware consisting of the following compositions (inweight): 65% of glass ceramic according to claim 2 having the followingcomposition:SiO₂—CaO—ZrO₂—ZnO (52.3:21.9:16.4:9.4) and 35% of a mixture oftraditional material consisting essentially of SiO₂ 60.0 wt. %; Al₂O₃37.0 wt. %; MgO 0.2 wt. %; CaO 0.8 wt. %; Na₂O 1.0 wt. % and K₂O 1.0 wt.%.
 15. Porcelain stoneware consisting of the following compositions (inweight): 5% of a glass ceramic according to claim 2 having the followingcomposition:SiO₂—CaO—ZrO₂—BaO—MgO (55:11:12:11:11) and 95% of a mixture oftraditional material consisting essentially of SiO₂ 72.2 wt. %; Al₂O₃18.5 wt. %; MgO 0.9 wt. %; CaO 1.3 wt. %; Na₂O 4.0 wt. % and K₂O 3.1 wt.%.
 16. Porcelain stoneware consisting of the following compositions (inweight): 50% of glass ceramic according to claim 2 having the followingcomposition:SiO₂—CaO—ZrO₂—BaO—MgO (55:11:12:11:11) and 50% of a mixture oftraditional material consisting essentially of SiO₂ 81.5 wt. %; Al₂O₃12.0 wt. %; MgO 0.2 wt. %; CaO 1.0 wt. %; Na₂O 2.2 wt. % and K₂O 3.1 wt.%.
 17. Porcelain stoneware consisting of the following compositions (inweight): 65% of glass ceramic according to claim 2 having the followingcomposition:SiO₂—CaO—ZrO₂—BaO—MgO (55:11:12:11:11) and 35% of a material consistingessentially of SiO₂ 58.5 wt. %; Al₂O₃ 37.3 wt. %; MgO 0.3 wt. %; CaO 0.6wt. %; Na₂O 1.0 wt. % and K₂O 2.3 wt. %.
 18. Ceramic materialsconsisting of the glass-ceramics according to claim
 1. 19. Ceramicmaterials consisting of the glass-ceramics according to claim
 4. 20.Ceramic materials consisting of the glass-ceramics according to claims6-17.